1. Field of the Invention
The preferred embodiments relate to well logging. More particularly, the preferred embodiments relate to generating a downhole clock for improving data measurements taken during logging.
2. Description of the Related Art
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information may include characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” may be performed by several methods.
In conventional wireline logging, a probe or “sonde,” housing formation sensors, may be lowered into the borehole after some or all of the well has been drilled. Once in the borehole, the sonde may be used to determine characteristics of the borehole, as well as formations traversed by the borehole. The upper end of the sonde may be attached to a wireline that suspends the sonde in the borehole. Power may be transmitted to the sensors and instrumentation in the sonde through the wireline. Similarly, the instrumentation in the sonde may communicate information to the surface by electrical signals transmitted through the wireline.
An alternative method of logging involves the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing or tripping the drilling assembly to insert a wireline logging tool. Accordingly, drilling techniques may be modified during drilling in order to optimize performance while minimizing down time. Measuring conditions downhole, including the movement and location of the drilling assembly, contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD.” Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as “logging-while-drilling” techniques, or “LWD.” For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Sensors or transducers may be located at the lower end of the drillstring in LWD systems. During drilling, these sensors may continuously or intermittently monitor drilling parameters and formation data. In some circumstances, energy sources for measurements, such as acoustic noise sources, may be located proximate to sensors on the drillstring. In other circumstances, the energy sources may be located elsewhere, such as seismic sources at the surface or within other boreholes. In order to retrieve useful information from received signals, it may be necessary to measure the elapsed time between when the signal emanates from the source, and reception by the downhole sensor. Thus, each datum received may be associated with an instant of time—i.e., time-stamped—based on a downhole clock in LWD operations.
If the source energy originated from downhole, the transmission time and arrival time may be correlated by the same downhole clock. In some systems, however, the energy may originate from the surface (or other location), and a second surface clock may also be used. As a consequence of using two clocks, synchronization between the two clocks may be important in order to obtain accurate LWD data. The two clocks may be in a non-synchronous state for a variety of reasons. For example, the surface clock may be a highly accurate time source, such as a GPS disciplined clock, while the downhole clock may vary according to its operating conditions, such as downhole temperature.
U.S. patent application number 2002/0125966A1 to Gunawardana et al. (hereinafter '966 patent) may disclose a downhole clock with improved temperature behavior. The clock in the '966 patent may be based on a single crystal that operates simultaneously in two modes of oscillation. A first mode of oscillation produces a frequency that may be somewhat independent of temperature, and a second mode of oscillation produces a frequency that may be somewhat dependent on temperature. The ratio between the frequencies over a range of temperatures may be used to compensate for fluctuations in the oscillation frequency of the first mode of oscillation. Because a single crystal is used, the temperature sensitivity of the oscillation frequencies may be inadequate for their intended purpose. For example, the difference, with respect to temperature, between the frequency associated with the first mode of oscillation and the frequency associated with the second mode of oscillation may be too small to provide the required level of sensitivity to accurately compensate changes in frequency.